1. Field of the Invention
This invention relates generally to a tensioner for tensioning a belt of a power transmission belt drive system. More particularly, it relates to a tensioner having dual arcuate moving pulleys and a strut as a biasing or bias-modifying member. Specifically, this invention relates to a dual arcuate movement strut tensioner having a geometry that reduces imbalances across the pivots at the centers of the arcuate movements.
2. Description of the Prior Art
Power transmission belt tensioners used in conjunction with power transmission accessory belt drive systems for internal combustion engines are known with pulley movements that are arcuate and include a strut for biasing or modifying the bias of the pulley which rests against the belt and thus controls tension of the belt drive of the system. Commonly, the pulley, the pivot about which the body of the tensioner rotates in operation and the attachment point of the strut are all coplanar. An example of this coplanar arrangement can be found in U.S. Pat. No. 5,439,420, to Meckstroth, et al.
It is also known to use tensioners having dual pulleys. The dual pulley tensioner design is particularly applicable to accessory belt drive systems having accessories that are hard to manage. An example is found in U.S. Pat. No. 4,416,674 to White Jr. where a dual pulley tensioner is being applied to cope with the hard to manage accessory in the form of a compressor for an air conditioner. An accessory that tends to be more problematic than the compressor of an air conditioner is the starter/generator. The starter/generator is a unitary device the performs both the function of the traditional starter and the function of the traditional starter/generator and is included in the front-end accessory drive of the engine. While, the instant invention is applicable to any power transmission belt drive system. It is particularly useful for those systems that include a starter/generator.
However, certain geometries of power transmission accessory belt systems do not lend themselves to this coplanar configuration. When such a geometry is present and it is desired to use a tensioner employing a strut, it becomes necessary to laterally offset the pulley in relation to the pivot to provide the needed clearances. An example where the pulley is offset in this manner can be found in U.S. Pat. No. 6,039,664, to Schmid. It can be seen in the '664 patent that the plane at the center of rotation of the pulley, or the plane that bisects the pulley, is offset from the plane of rotation at the center of the pivot bearing. Still, the attachment point for the strut remains substantially in the plane at the center of rotation of the pivot bearing. It is readily apparent that, when the subject tensioner is placed into operation, the forces acting upon the tensioner by the belt and the strut give rise to an unbalanced load across the pivot bearing. This would be true whether the strut is providing the biasing force for the tensioner or only modifying the biasing force through damping. An unbalanced load causes parasitic torque, which tends to force the pivot shaft to axially misaligned with the bearing. Such parasitic torque in combination with normal rotational movement tends to cause uneven wear in bearings. If uneven wear is allowed to progress, a bearing will exhibit undesirable properties such a looseness, noise, misalignment, and failure.
Accordingly, there is a continuing need for an arcuate movement dual pulley tensioner utilizing the beneficial properties of a strut biasing member and offset to accommodate certain power transmission accessory belt drive system geometries that operates without substantial parasitic torque for improved bearing life properties.